1. Field of the Invention
The present invention relates to a method for applying a coating composition and particularly to a so-called extrusion coating method in which a leading end of a coating head is disposed so as to be opposite to a surface of a continuously conveying flexible support supported by conveying guide means such as guide rollers, etc. so that a coating composition extruded from a leading end of a slit of the coating head is applied onto the surface of the flexible support.
2. Description of the Related Art
Heretofore, a process for producing a photographic light-sensitive material, a magnetic recording medium, or the like, contains a step of applying a desired coating composition onto a flexible support. Various liquid compositions are used as the coating composition in accordance with purposes of use. Examples of the coating composition include coating compositions for a light-sensitive emulsion layer, an undercoat layer, a protection layer, a back layer, etc. in a photographic light-sensitive material, and coating compositions for a magnetic layer, an undercoat layer, a protection layer, a lubricant layer, etc. in a magnetic recording medium. Each of these coating compositions is an aqueous or organic solvent solution containing an essential component, a binder, and various additives if necessary.
Various methods such as a roller coating method, a gravure coating method, a roller coating and doctor coating method, an extrusion coating method, a slide coating method, etc. have been heretofore used as methods for applying the aforementioned coating composition onto a surface of a flexible support. The extrusion coating method is used popularly for application of a magnetic coating composition in recent years.
As known well, the extrusion coating method is a coating method in which a coating head having a slit is pressed against a continuously conveying flexible support so that a coating composition extruded from the slit in a doctor edge portion is applied thinly and uniformly onto a surface of the flexible support. Various discussions have been heretofore made upon the shape of the doctor edge portion, the pressure of the coating composition, the positional relation between the support and the leading end of the doctor edge, etc. as disclosed, for example, in Japanese Patent Unexamined Patent Publication No. Sho. 60-238179, Japanese Patent Unexamined Patent Publication No. Sho. 62-132566, Japanese Patent Unexamined Patent Publication No. Hei. 4-145977, etc.
The extrusion coating method has a problem in that the coating thickness becomes uneven in the direction of the width of the flexible support. This unevenness is established when the sum P of the pressure loss p1 in the passage of the coating composition through the slit and the pressure p2 generated on the doctor edge surface on the basis of pressing of the flexible support is uneven in the direction of the width of the support. When, for example, the slit gap or the shape of the doctor edge surface is uneven in producing accuracy or thickness irregularity, partial deformation (roughness), or the like, occurs in the direction of the width of the flexible support, unevenness of coating thickness occurs in the direction of the width of the support.
To solve this problem, coating apparatuses, or the like, each provided with adjusting means for adjusting slit gaps individually in the direction of the width of the support are proposed as disclosed in Japanese Patent Examined Patent Publication No. Hei. 7-106332 and Japanese Patent Unexamined Patent Publication No. Hei. 5-293419. In the proposed apparatuses, a distribution in the direction of the width of the support is given to the pressure loss in the passage of the coating composition through the slit to thereby improve the irregularity of coating thickness caused by the unevenness of the support.
On the other hand, as the trend of magnetic recording media in recent years, the thickness of a magnetic layer conducting recording has been reduced with the demand on high-density recording media. As the coating thickness decreases, more accurate thickness is required. Accordingly, the aforementioned thickness adjusting mechanism becomes essential.
However, when such an extrusion coating head is used for a flexible support having a high toughness, the pressure p2 generated on the doctor edge surface is higher than the pressure loss p1 in the slit. Accordingly, there arises a problem that the sum P of pressures does not change sufficiently even in the case where the pressure loss p1 is changed by use of the aforementioned adjustment means for adjusting the coating thickness in the direction of the width of the support. Furthermore, to widen the slit gap distribution more greatly, it is necessary to deform the coating head while exceeding the elastic deformation limit of the coating head material. This is undesirable for use.
An object of the present invention is to provide a coating method and an extrusion coating head for the same in which coating thickness can be adjusted in a small quantity of deformation within a range of the elastic deformation limit of the material of an extrusion coating head to solve the aforementioned problems in the prior art.
The above-mentioned object can be attained by an extrusion coating method, according to the present invention, comprising the steps of:
providing an extrusion coating head extending in a width direction perpendicular to a conveying direction of a continuously conveying flexible support and including an upstream side block and a downstream side block aligned in the conveying direction, in which the upstream and downstream side block respectively have edge surfaces on which the flexible support is pressed and the upstream and downstream side block define therebetween a slit through which a coating composition can be extruded and applied onto the flexible support; and
adjusting a length (t) of a perpendicular line which is drawn from an upstream end (A) of the edge surface of the downstream side block to a tangential line (L) of the edge surface of the downstream side block which is drawn from a downstream end of the edge surface of the upstream side block, to thereby adjust a distribution of a coating thickness on the flexible support in the width direction.
In the above-mentioned extrusion coating method, it is preferable to further include the steps of:
adjusting a first length (t) of a perpendicular line which is located in a first plane which extends in the conveying direction; and
adjusting a second length (t) of a perpendicular line which is located in a second plane which extends in the conveying direction, the second plane being different from the first plane,
in which each of the first and second lengths is defined by a perpendicular line which is drawn from an upstream end (A) of the edge surface of the downstream side block to a tangential line (L) of the edge surface of the downstream side block which is drawn from a downstream end of the upstream side block, to thereby partially adjust a distribution of a coating thickness on the flexible support in the width direction.
In addition, in the above-mentioned extrusion coating method, it is preferable that the upstream side block is a back edge block and the downstream side block is a doctor edge block when the extrusion coating method is used for a coating method providing one layer on the flexible support.
Further, in the above-mentioned extrusion coating method, it is preferable that the upstream side block is a lower-layer-side doctor edge block and the downstream side block is an upper-layer-side doctor edge block when the extrusion coating method is used for a coating method providing multi-layers on the flexible support.
Moreover, in the above-mentioned extrusion coating method, it is preferable that the length adjusting step comprises the steps of:
inclining the downstream side block relative to the conveying direction in such a manner that an angle (xcex8) defined between the tangential line L and a center axis of the slit is not larger than 90xc2x0.
Furthermore, in the above-mentioned extrusion coating method, it is preferable that the length adjusting step comprises the steps of:
inclining the downstream side block relative to the conveying direction in such a manner that an angle (xcex8) defined between the tangential line L and a center axis of the slit is in the range of 45xc2x0 to 80xc2x0.
In addition, the above-mentioned object can also be achieved by an extrusion coating head, according to the present invention, comprising:
an upstream side block and a downstream side block aligned in the conveying direction, in which the upstream and downstream side block respectively have edge surfaces on which the flexible support is pressed and the upstream and downstream side block define therebetween a slit through which a coating composition can be extruded and applied onto the flexible support; and
an adjusting mechanism for adjusting a length (t) of a perpendicular line which is drawn from an upstream end (A) of the edge surface of the downstream side block to a tangential line (L) of the edge surface of the downstream side block which is drawn from a downstream end of the edge surface of the upstream side block.
In the above-mentioned extrusion coating head and method, it is preferable that the upstream side block is a back edge block and the downstream side block is a doctor edge block when the extrusion coating method is used for a coating method providing one-layer on the flexible support.
In the above-mentioned extrusion coating head and method, it is preferable that the upstream side block is a lower-layer-side doctor edge block and the downstream side block is an upper-layer-side doctor edge block when the extrusion coating method is used for a coating method providing multi-layers on the flexible support.
In the above-mentioned extrusion coating head, it is preferable that the length adjusting mechanism comprises:
an inclining means for inclining the downstream side block relative to the conveying direction.
In addition, in the above-mentioned extrusion coating head, it is preferable that the inclining means inclines the downstream side block in such a manner that an angle (xcex8) defined between the tangential line L and a center axis of the slit is not larger than 90xc2x0.
Further, in the above-mentioned extrusion coating head, it is preferable that the inclining means inclines the downstream side block in such a manner that an angle (xcex8) defined between the tangential line L and a center axis of the slit is in the range of 45xc2x0 to 80xc2x0.
Furthermore, in the above-mentioned extrusion coating head, it is preferable to further include:
a plate member disposed under the downstream side block; and
a flat plate interposed between the downstream side block and the plate member so as to allow the downstream side block to be elastically and partially deformed in the width direction,
wherein the adjusting mechanism comprises a screw member for attaching the downstream side block to the plate member through the flat plate and simultaneously adjusting the elastically deforming condition of the downstream side block.
In the above-mentioned construction, it is advantageous that the plate member is shorter than the downstream side block in the width direction.
In the above-mentioned construction, it is advantageous that the adjusting mechanism comprises:
a plurality of adjusting members which are aligned in the width direction and each adjusts the length (t) in respective cross sections.
In the above-mentioned construction, it is advantageous that the adjusting member comprises:
L-shaped plate having a plurality of holes aligned in the width direction, and
a plurality of screws threadely engaged with respective holes of said L-shaped plate.
Note that, although the length t may be off-line adjusted, the length t can be on-line adjusted if the coating head is provided with adjusting mechanisms.